--- a/mail/themes/linux/mail/compose/messengercompose.css
+++ b/mail/themes/linux/mail/compose/messengercompose.css
@@ -249,17 +249,17 @@ toolbar[brighttext] menulist > .menulist
 }
 
 #msgSubject {
   -moz-appearance: none;
   margin-top: 0;
   margin-inline-start: 6px;
   margin-inline-end: 5px;
   background-color: inherit;
-  border-color: transparent;
+  border: 2px solid transparent;
   -moz-border-top-colors: none;
   -moz-border-left-colors: none;
   -moz-border-right-colors: none;
   -moz-border-bottom-colors: ThreeDShadow ThreeDHighlight;
   padding: 2px;
   padding-inline-start: 5px !important;
   transition: border .2s, background-color .2s;
 }

--- a/mail/themes/windows/mail/compose/messengercompose.css
+++ b/mail/themes/windows/mail/compose/messengercompose.css
@@ -127,39 +127,43 @@
   display: none;
 }
 
 #msgSubject {
   -moz-appearance: none;
   margin-top: 0;
   margin-inline-end: 5px;
   background-color: inherit;
+  border-style: solid;
   border-color: transparent;
   -moz-border-top-colors: none;
   -moz-border-left-colors: none;
   -moz-border-right-colors: none;
   -moz-border-bottom-colors: none;
   padding: 2px;
   padding-inline-start: 5px;
   transition: border .2s, background-color .2s;
 }
 
 @media (-moz-windows-default-theme) {
   #msgSubject {
     border-width: 1px;
+    border-bottom-color: #a9b7c9;
   }
 
   #msgSubject:hover,
   #msgSubject[focused="true"] {
     background-color: -moz-field;
+    border-color: #a9b7c9;
   }
 }
 
 @media (-moz-windows-default-theme: 0) {
   #msgSubject {
+    border-width: 2px;
     -moz-border-bottom-colors: ThreeDShadow ThreeDHighlight;
   }
 
   #msgSubject:hover,
   #msgSubject[focused="true"] {
     background-color: -moz-field;
     -moz-border-top-colors: ThreeDShadow ThreeDHighlight;
     -moz-border-right-colors: ThreeDShadow ThreeDHighlight;
@@ -791,27 +795,16 @@ toolbar[brighttext] #button-print {
   border-bottom-width: 0;
   background-color: transparent;
 }
 
 #subjectLabel {
   margin-top: 0;
 }
 
-@media (-moz-windows-default-theme) {
-  #msgSubject {
-    border-bottom-color: #a9b7c9;
-  }
-
-  #msgSubject:hover,
-  #msgSubject[focused="true"] {
-    border-color: #a9b7c9;
-  }
-}
-
 /* ::::: autocomplete icons ::::: */
 
 treechildren::-moz-tree-image(local-abook) {
   margin-inline-start: 2px;
   margin-inline-end: 5px;
   list-style-image: url("chrome://messenger/skin/addressbook/icons/addrbook.png");
 }